feat(workspace): journal panel toggle, view kept across tabs, sections nest under headings

- the journal opens as a framed panel and closes on a second click,
  returning to the previous note; journal state and the last regular note
  survive tab switches, the fallback note is never a journal day
- pasted and imported markdown nests content under its heading (H2 below
  H1), Enter on a heading creates a child, '- ' / '* ' / '+ ' start a bullet

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/storage/import_markdown.go

@@ -740,9 +740,10 @@ func indentWidth(s string) int {
 func MarkdownToBlocks(md string) []domain.DocumentBlock {
 	lines := strings.Split(md, "\n")
 	var blocks []domain.DocumentBlock
-	base := 0         // 1 inside a heading section
-	var indents []int // indent widths of the open list levels
-	inList := false   // the previous line belonged to a list item (for continuation lines)
+	base := 0          // level of content in the current heading section
+	var headings []int // ranks (1–3) of the open headings: H2 below H1 nests one level deeper
+	var indents []int  // indent widths of the open list levels
+	inList := false    // the previous line belonged to a list item (for continuation lines)
 	listLevel := func(width int) int {
 		for len(indents) > 0 && indents[len(indents)-1] > width {
 			indents = indents[:len(indents)-1]
@@ -789,15 +790,23 @@ func MarkdownToBlocks(md string) []domain.DocumentBlock {
 			if typ == "" {
 				typ = "h3"
 			}
-			blocks = append(blocks, newBlock(typ, strings.TrimSpace(m[2]), 0))
-			base, indents, inList = 1, nil, false
+			rank := len(m[1])
+			if rank > 3 {
+				rank = 3
+			}
+			for len(headings) > 0 && headings[len(headings)-1] >= rank {
+				headings = headings[:len(headings)-1]
+			}
+			blocks = append(blocks, newBlock(typ, strings.TrimSpace(m[2]), len(headings)))
+			headings = append(headings, rank)
+			base, indents, inList = len(headings), nil, false
 			continue
 		}
 
 		if trimmed == "---" || trimmed == "***" || trimmed == "___" {
 			if !strings.HasPrefix(line, " ") && !strings.HasPrefix(line, "\t") {
 				blocks = append(blocks, newBlock("divider", "", 0))
-				base, indents, inList = 0, nil, false
+				base, headings, indents, inList = 0, nil, nil, false
 				continue
 			}
 		}